1. Field of the Invention
The present invention relates to a multilayer piezoelectric element. More precisely, the present invention relates to a multilayer piezoelectric element which can prevent a characteristic degradation caused by a pyroelectric effect.
2. Description of the Related Art
The piezoelectric element is an element, which mutually transforms between mechanical displacement and electric displacement by using piezoelectric effect and inverse piezoelectric effect. Such piezoelectric elements are manufactured e.g. by forming and firing a piezoelectric ceramic to obtain element body, forming electrode thereto, and further performing polarization treatment.
The mechanical displacement caused by the piezoelectric element is relatively small; and that the piezoelectric element is suitably used for such as an actuator in which accuracy and exactness is required. In concrete, it is used for such as a lens driver, a head driver of HDD, a head driver for ink-jet printer, a fuel injection valve driver, and the like.
These piezoelectric elements, when used in portable electronic devices and the like, will be exposed to a temperature change caused by surrounding environmental changes.
When assumed to be exposed to such temperature change, a problem such as deterioration of polarization degree due to pyroelectric effect may occur to piezoelectric element. Particularly, in the course of temperature decrease, a voltage will be applied to a piezoelectric element in an inverse direction of a voltage applied during polarization treatment by charges due to pyroelectric effect; and that polarization degree of the piezoelectric element is likely to be decreased when said voltage is too large. There is little hope for the polarization degree decreased in such way to recover in the course of temperature increase, and that polarization degree shows gradual decline as temperature changes repeatedly. As a consequence, it causes a problem of piezoelectric element characteristic degradation, such as not obtaining a desired displacement.
In order to handle such problems, Patent Article 1 (International Laid-Open Patent No. 2007/052599) describes a technique wherein an internal electrode exposed to a side surface of multilayer piezoelectric element is covered with an exterior material for preventing migration, and a conducting particle is dispersed to the exterior material.
In addition, Patent Article 1 also describes that decrease of polarization degree due to pyroelectric effect can be inhibited by the conducting particle dispersed exterior material.